Central memory T (TCM) cells in lymph nodes (LN) and resident memory T (TRM) cells in peripheral tissues play distinct roles in protective immunity1-5. Both are generated after primary infections, but the clonal origin of TRM and TCM cells is unclear. To address this question, mice were immunized through the skin with either a protein antigen, a chemical hapten, or a non-replicating poxvirus. We then analyzed antigen activated T cells from different tissues using high-throughput sequencing (HTS) of the gene (Tcrbv) encoding T cell receptor gene β chain CDR3 region to simultaneously track thousands of unique T cells6. For every abundant TRM clone generated in the skin, an abundant TCM clone bearing the identical TCR was present in lymph nodes (LN). Thus antigen reactive skin TRM and LN TCM clones were derived from a common naive T cell precursor after skin immunization, generating overlapping TCR repertoires. Although they bore the same TCR, TRM mediated rapid contact hypersensitivity (CHS)7 responses in mice, whereas TCM mediated delayed and attenuated responses. Studies in human subjects confirmed the generation of skin TRM in allergic contact dermatitis. Thus, immunization through skin simultaneously generates skin TRM and LN TCM in similar numbers from the same naïve T cells.

Smc5/6 is a structural maintenance of chromosomes complex, related to the cohesin and condensin complexes. Recent studies implicate Smc5/6 as being essential for homologous recombination. Each gene is essential, but hypomorphic alleles are defective in the repair of a diverse array of lesions. A particular allele of smc6 (smc6-74) is suppressed by overexpression of Brc1, a six-BRCT domain protein that is required for DNA repair during S-phase. This suppression requires the postreplication repair (PRR) protein Rhp18 and the structure-specific endonucleases Slx1/4 and Mus81/Eme1. However, we show here that the contribution of Rhp18 is via a novel pathway that is independent of PCNA ubiquitination and PRR. Moreover, we identify Exo1 as an additional nuclease required for Brc1-mediated suppression of smc6-74, independent of mismatch repair. Further, the Apn2 endonuclease is required for the viability of smc6 mutants without extrinsic DNA damage, although this is not due to a defect in base excision repair. Several nucleotide excision repair genes are similarly shown to ensure viability of smc6 mutants. The requirement for excision factors for the viability of smc6 mutants is consistent with an inability to respond to spontaneous lesions by Smc5/6-dependent recombination.

CCR7−/− mice exhibit profound anomalies in LN and spleen architecture, which complicates the study of CCR7-mediated T cell trafficking in vivo. To circumvent this problem, we established in vivo models in which WT and CCR7−/− populations coexist within mice possessing normal lymphoid organs, and must compete for developmental niches within the tissues of these mice. Under the conditions we have created in vivo, we find the entry of memory CD4 T cells into LN from the blood to be independent of CCR7. Thus, the central memory CD4 T cells that traffic though LN, which are often defined by their expression of CCR7, do not appear to gain any competitive homing advantage by expressing this receptor. Furthermore, in contrast to cutaneous DC populations, we found that CCR7 deficiency had no appreciable effect on the exit of CD4 T cells from inflamed skin. Finally, we found that WT and CCR7−/− precursors were equally represented within the major thymic subpopulations, despite previous findings that CCR7 plays a role in seeding the thymus from bone marrow-derived T cell precursors.

Topical antigen application is a focus of current vaccine research. This immunization route mimics natural antigen exposure across a barrier tissue and generates T cells imprinted for skin-selective homing. Soluble antigens introduced through this route require cross-presentation by DC to generate CD8 T cell responses. Here we have explored the relative contribution of various skin-derived DC subsets to cross-priming and skin-selective imprinting. In our model, DC acquire soluble Ag in vivo from immunized murine skin for cross-presentation to naïve CD8 T cells ex vivo. We find CD11b+ migratory DC to be the relevant cross-priming DC in this model. Both Langerin+ and Langerin- CD11b+ migratory DC can cross-present antigen in our system, but only the Langerin+ subset can induce expression of the skin-selective addressin E-selectin ligand. Thus, the CD11b+ Langerin+ migratory DC population, comprised primarily of Langerhans cells, both cross-primes naïve CD8 T cells and imprints them with skin-homing capabilities.